Pararrhopalia Simroth, 1893

Salvini-Plawen, Luitfried von, 2008, Contributions to West European Cavibelonia (Mollusca, Solenogastres) with two new species, Zoosystema 30 (4), pp. 873-897 : 884-886

Genus Pararrhopalia Simroth, 1893

TYPE SPECIES. — Pararrhopalia pruvoti Simroth, 1893 . DIAGNOSIS. — Solenogastres with thick cuticle and hollow sclerites in more than one layer, also including hooked ones; mouth opening separated from atrial sense organ; distichous radula present; with dorsal papillose foregut gland; unpaired secondary genital opening, with copulatory stylets; with dorsoterminal sense organ, without respiratory organs.

Pararrhopalia pruvoti Simroth, 1893 ( Figs 10-12 )

Pararrhopalia pruvoti Simroth, 1893a: 325 .

Proneomenia vagans – Pruvot 1891: 723 (not Proneomenia vagans Kowalevsky & Marion, 1887 ).

MATERIAL EXAMINED. — Specimens collected during the Spanish DIVA-Artabria I project in September 2003 off Galicia ( Spain) were used by Christiane Todt for special investigations (Todt 2006). 2 specimens from about 43°33’N, 8°35’W, at 150 m ; 1 individual (5.5 mm) not clearly conspecific from about 43°48’N, 8°53’W, at 600 m. Serial semithin (cs 2 µm) sections stained by Richardson’s solution, and ultrathin sections.

DIAGNOSIS. — Up to 5 mm long. Mantle with intercrossing tangential spicules plus radial spicules. With small pre-atrial sense organ, atrial sense organ terminally bilobed; Radula teeth with distal hook and 2 or 3 median denticles. With oesophagus; midgut caecum anteriorly paired, without regular midgut pouchings. With one pair of seminal vesicles, no receptacula seminis; fused part of spawning ducts with a paired, posteriorly directed lateral pouch; copulatory apparatus paired, each with a bundle of stylets in body axis.

DESCRIPTION

External appearance

Specimens 3 mm long. Body round, without keel; body outline uneven due to radial spicules ( Fig. 10 ).

Mantle

Cuticle 35-40 µm, dorsally up to 60 µm. Sclerites ( Fig. 11 ; not figured by Pruvot 1891, but referred to the sclerites of Pruvotina impexa (Pruvot, 1890) are hollow tangental spicules intercrossing in three layers, with several (somewhat obliquely positioned) radial spicules extending far beyond cuticle, all 80-120 × 5-8 µm; along pedal groove, knife-blade scales (35-45 × 8 µm) and adjoining slender needles (90-120 × 3-5 µm); radial, hooked sclerites (about 40 µm long) dorsally, more dense posteriorly. Pluricellular epidermis papillae distinct.

Foot and mantle cavity

One pedal fold reaching the ciliated, narrow mantle cavity which lacks respiratory organs.

Sensory system

Unpaired cerebral ganglion with three pairs of frontal nerves with distal swelling above atrium. Buccal ganglia small (diam. about 20 µm), to each side of pharynx. Medullar suprarectal commissure at end of pericardium about 50 µm long.

Atrial sense organ terminally bilobed and delimited by a horseshoe-shaped ciliary tract, middorsally fused for a short distance only; most papillae of atrium basally bundled into groups of three. With circular (sense) organ close to anterior rim of atrial sense organ, and with terminally located dorsoterminal sense organ.

Alimentary tract

Mouth opening distinctly separated from atrium; anterior pharynx with strong circular musculature, behind this a narrow dorsal pouch includes the papilla of the dorsal foregut gland (cf. Todt 2006). Radula teeth about 25 µm long with distal hook and 2 or 3 median denticles; ventral foregut glandular organs with subepithelially positioned glands opening into a paired duct (type A in Salvini-Plawen 1972, 1978), these ducts opening separately below radula. Distinct post-radular foregut (oesophagus) opening ventrofrontally into midgut. Midgut caecum anteriorly paired. Dorsoventral muscle bundles not causing distinct regular lateral pouchings of the midgut due to the wide distance of the gut to the body wall; middorsal ciliary tract present.

Gonopericardial system

Pericardium wide with heart ventricle as a free tube, auricle as dorsal invagination. Smaller specimens with one posteriorly directed, lateral seminal vesicle at the beginning curve of each pericardioduct, the latter opening dorsolaterally into spawning ducts; no receptacula seminis. Spawning ducts paired for a short distance only; more posteriorly the single spawning duct exhibits a paired, posteriorly directed lateral pouch, which makes the organ three-parted ( Fig. 12 ), with the central duct portion opening ventrofrontally into the mantle cavity.Paired copulatory stylet apparatus with a bundle of 6-8 stylets each ( Fig. 12 ), proximally with muscular sheath, opening laterally of terminal pedal fold at rim of mantle cavity opening.

DISCUSSION

The organization of the present specimens (hollow, hooked sclerites; papillose dorsal foregut gland, etc.) clearly corresponds to the Pruvotinidae and all essential characters coincide with Pararrhopalia Simroth, 1893 (see Salvini-Plawen 2003b). Two species are known, the type species P. pruvoti (misidentfied as “ Proneomenia vagans ” in Pruvot 1891 ) from off Banyuls-sur-Mer (S France) and P. (?) fasciata Salvini-Plawen, 1978, from the South Shetland Islands. Several characters differ in P. (?) fasciata (no oesophagus, short midgut caecum unpaired, with receptacula seminis, copulatory stylets in transversal position). A detailed comparison of the new 3 mm specimens with Pruvot’s (1891) description of the type specimen (5 mm) revealed a clear correspondence (e.g., Pruvot 1891: fig. 55 and Todt 2006: fig. 6A). This holds true also for the pre-atrial sense organ (see Pruvot 1891: fig. 86) and in particular for the configuration of the spawning ducts and the bundles of 5-7 copulatory stylets (see Pruvot 1891: figs 60, 60a). The pouchings along the pericardioducts – to form multiple vesiculae seminales – in Pruvot’s specimen ( Pruvot 1891: 772, fig. 60) obviously refer to full-grown animals only and are absent in the smaller specimen investigated here (150 m depth, 3 mm long; eggs still in development with diam. 10-20 µm). Hence, the present animals are conspecific and represent the first findings after the original description of Pararrhopalia pruvoti . They thus enlarge the geographical range of this species to the NW-Iberian Atlantic.

The larger, 5.5 mm long, specimen shows an identical organization of the anterior body. The posterior body is somewhat damaged terminally and the paired bundle of 12 copulatory stylets opening laterally of terminal pedal fold is the only coincidence; there are no discernible eggs, the characteristic tripartition of the spawning duct(s) is at most indicated and other organs are not satisfactorily visible. Consequently, this specimen from 600 m depth cannot be definitively attributed to P. pruvoti .

Figures

FIG. 10.— Pararrhopalia pruvoti Simroth,1893,preserved specimen, 3 mm long (anterior end above).

FIG. 11. — Pararrhopalia pruvoti Simroth, 1893, mantle sclerites. Scale bar: 20 μm.

FIG. 12. — Pararrhopalia pruvoti Simroth, 1893, cross section through posterior body showing the three-parted spawning duct due to a paired,posteriorly directed lateral pouch. Abbreviations:co, supra-rectal commissure;cs, bundle of copulatory stylets;hg, hindgut; pd, pericardioduct (posterior curve); sd, spawning duct; sdp, pouch of spawning duct. Scale bar: 50 μm.